When an exhaust gas purification system uses a NOx storage reduction catalyst or a lean NOx trap (hereinafter, referred to as “LNT catalyst”), sulfur dioxide in an exhaust gas, which is primarily derived from fuel, is absorbed by the LNT catalyst and a NOx purification rate decreases. In order to avoid such decrease in the NOx purification rate, it is necessary to periodically perform a desulfurization process (sulphur purge process). The desulfurization process brings the exhaust gas into a rich condition, i.e., a condition of low oxygen concentration, to remove sulfur dioxide, which is absorbed by the LNT catalyst.
The desulfurization process requires high temperature. Thus, if the desulfurization process is applied to a vehicle that has a DPF (diesel particulate filter) configured to collect PM (particulate matter) contained in an exhaust gas, the DPF regeneration process is usually carried out (i.e., PM collected by the DPF is burned and removed with the high temperature exhaust gas) prior to the desulfurization process.